The present invention relates to refractory bearings and is particularly concerned with refractory bearings for use in contact with molten metal.
Bearings typically support a rotating or otherwise moving article. In high temperature applications, bearings often comprise a refractory metal, ceramic or composite. In such applications, bearings may even be in direct contact with molten metals, such as molten zinc or aluminum.
For example, galvanization is the process of forming a protective, anti-oxidant zinc layer on a base metal. A continuous galvanizing apparatus comprises a bath of molten zinc, a sink roll at least partially immersed in the bath, a journal disposed along the longitudinal axis at each end of the sink roll around which the sink roll may rotate, and a set of roll arms with replaceable bearings for supporting the journals. The bath is maintained at a temperature sufficient to keep the zinc molten.
The sink roll forces the base metal, which is often in the form of a sheet or wire, into the molten zinc. The roll rotates as the base metal passes into the molten zinc, under the sink roll, and finally out of the molten zinc. The journals cooperate and rotate with the sink roll. The roll arms support the journals, and the journals are often covered with a hard material, such as tungsten carbide, to resist wear from the bearing. The roll arm can be moved to adjust the depth of the sink roll within the bath.
The requirements for bearings under these conditions are severe. Molten zinc is at least 420xc2x0 C., typically around 460xc2x0 C., and corrodes many common bearing materials. Mechanical abrasion is a ubiquitous complication. The bearings wear quickly and must be replaced frequently. Replacement requires the shutdown of the galvanizing operation while new bearings are inserted. Disruption of a continuous galvanizing operation results in significant operator costs and lost production.
Prior art bearings include metal housings, often in the shape of a ring, that are fitted to the roll arm and cooperate with the journals. During operation of a galvanizing bath, the journals contact a working face of the bearing. The high temperature and corrosive environment destroy metal bearings relatively quickly and cause the sink roll to rotate eccentrically, thereby reducing galvanizing efficiency. Worn bearings must be replaced, often at great cost. Prior art also includes metal housings having inserts comprising a refractory material selected for its erosion and corrosion resistance. The journals contact the inserts instead of the metal housing. The inserts are substantially more resistant to wear and corrosion than the metal housing alone and can extend the life of bearings many times.
Sialon is particularly useful in this capacity and consists of a solid solution and/or dispersion of aluminum oxide and aluminum nitride in a silicon nitride matrix. One or more sialon inserts are embedded into the working face of the metal housing. Typically, the inserts are polygonal shapes and are embedded in a plurality of cavities along the working face of the metal housing.
At room temperature, the inserts are secured tightly into the cavities. This may be accomplished using a retaining plate and one or more wedges to improve the tightness of fit. The retaining plate can be welded to the housing and may extend at least partially over the insert in the cavity. Still, inserts have a tendency to fall out at operating temperatures because the thermal expansion of the metal housing is greater than the ceramic inserts. Loss of an insert causes the journal to wobble or otherwise rotate eccentrically. Fortunately, the journals pressing against the inserts can hold the inserts in the cavities despite thermal expansion; however, pressure can be lost when the galvanizing operation is stopped or slowed. In such situations, the journal may separate from the insert by one-quarter inch or more. An insert can then slip from its cavity. The tendency of a ceramic insert to loosen and fall from a metal cavity increases with temperature and would be even more likely at higher temperatures, such as with molten aluminum baths which are typically at least around 700xc2x0 C. and more commonly around 715xc2x0 C.
A need persists for a refractory bearing comprising a housing and a wear-resistant insert where the insert is more fixedly secured to the housing and does not depend on the journals to hold it in place. Advantageously, the bearing would be easily manufactured of substantially inexpensive materials and would be suitable for use with molten zinc and aluminum.
The objective of the present invention is to provide a bearing for high temperature applications, particularly for contact with molten metals. The invention secures a wear-resistant insert to the working surface of the bearing despite disparate dimensional changes caused by thermal expansion.
One aspect of the invention describes the bearing as comprising a housing having a wear-resistant insert at least partially embedded in a cavity of a working surface of the housing. A retainer is secured to the housing and simultaneously engages a relief in the insert, thereby locking the insert into the cavity.
A further aspect of the invention describes the housing as a metal ring adapted to receive a journal. The inner surface of the ring comprises the working surface, which includes a plurality of inserts axially arranged along the working surface. One embodiment of the invention includes a metal ring having three inserts along the working surface.
Another aspect of the invention teaches inserts comprising sialon. In a further aspect of the invention, the retainer may comprise one or more metal pieces and is conveniently welded to a metal housing.